I've done a lot of account sales and buys, etc. If you plan on buying an account you need to get the person's full EQ EMU info and go here:
http://www.eqemulator.org/
After logging in with the current login/password, click on the User CP panel and change the account password and then the account email. Next go to the email address you just changed it to and open up that email to confirm the email change. Finally, go into the Login server accounts and change the password.
It is best to only buy accounts when dealing with someone with a good rep on the forums or in-game. The buyer always hands in the full amount of plat up-front. So the buyer has to trust the seller 100%.